He was charged with one count of making a false statement to FBI agents.

WASHINGTON ― President Donald Trump's former national security adviser pleaded guilty Friday morning to lying to the FBI about his contacts with the Russian government, an extraordinary development in special counsel Robert Mueller's investigation into Russian interference in the 2016 election.

Michael Flynn was charged with one count of making a false statement to FBI agents and appeared in federal court in D.C.

In a criminal information filing from Mueller's team, the government alleges that Flynn "willfully and knowingly made materially false, fictitious, and fraudulent statements" in an interview with FBI agents on Jan. 24. It alleges he falsely told the FBI that he did not ask the Russian ambassador to refrain from retaliating to sanctions the Obama administration imposed on Russia in late December. Flynn also allegedly lied about asking the Russian ambassador on Dec. 22 to delay or defeat a pending United Nations Security Council resolution.

But those indictments involved only Trump campaign officials. Flynn is the first former administration official to be charged, making it more difficult for the White House to distance the president from Mueller's probe.

In the fall of 2016, a businessman with close ties to Turkey's autocratic President Recep Tayyip Erdogan paid Flynn more than $500,000 to conduct research aimed at discrediting the exiled cleric Fethullah Gülen. Flynn failed to disclose the work until March, after he had already stepped down as national security adviser.

Mueller's team has reportedly investigated Flynn's lobbying firm and its dealings with Turkey.

He also attended a meeting with Kislyak to discuss creating a backchannel with Putin. Also in attendance was Trump's adviser and son-in-law Jared Kushner, who is now a senior White House adviser and also reportedly a subject of Mueller's probe.

White House officials have repeatedly denied any collusion between Trump's team and Russia, often by downplaying suspected officials' involvement in the campaign or the administration. In March, then-press secretary Sean Spicer diminished Flynn as just "a volunteer."